Earlier in the week I plugged my ACK A-30 Altitude Encoder into the wiring harness to confirm that it was working. However when I turned on the transponder, it seemed that it was not communicating with the altitude encoder, as the pressure altitude field was blank. I pored over the installation manuals for both devices, but didn't find any clues. I finally got out the ohmmeter and checked to see which pins on the encoder end of the harness were connected to which pins on the transponder end. I discovered that two pins on the encoder end were miswired. Once I swapped the two pins the encoder and transponder started talking to each other. I checked the altimeter setting, and calculated the pressure altitude. The encoder must be calibrated later to meet the regulatory requirements, but right out of the box it seems to be within 100 ft of the correct reading.

I did do a check to make sure that 12 volts was only present on the one correct pin before I plugged the wiring harness into the encoder, but I didn't do any other checks on it. This is the second problem I have found with the wiring harness (there is also a glitch with the intercom harness that I have yet to fix). I'm not upset with the avionics vendor, as mistakes happen, and I know I have the ultimate responsibility for the quality of anything that I install in this aircraft. But I did learn that I should do a complete check of premade wiring harnesses, rather than just assuming that they were fabricated correctly.